Printers are useful for producing printed images of a wide range of types. Printers print on receivers (or “imaging substrates”), such as pieces or sheets of paper or other planar media, glass, fabric, metal, or other objects. Printers typically operate using subtractive color: a substantially reflective receiver is overcoated image-wise with cyan (C), magenta (M), yellow (Y), black (K), and other colorants. Prints can be produced with various surface finishes such as matte or glossy.
For security, watermarks are often provided on documents that should not be reproduced or counterfeited. A watermark is a pattern visible in the original document under some viewing conditions but not others. For example, cylinder-mold and dandy-roll watermarks vary the thickness of the paper in a pattern corresponding to the watermark. Thinner areas of the paper permit more light to pass through than thicker areas of the paper, so the watermark is visible when backlit. However, the watermark is generally not visible when front-lit. The watermark is therefore not copyable by typical office copiers, flatbed scanners, or devices that image the piece to be copied under front-lit conditions.
However, conventional watermarks require custom paper. In an attempt to provide watermarks that can be produced on standard papers, various schemes have been proposed that modify the image data to be printed. For example, U.S. Patent Publication No. 2008/0192297 describes using anisotropic halftone structures with different orientations to render different parts of an image. This document describes providing different gloss characteristics between the parts of the image printed with the different halftone structures. U.S. Patent Publication No. 2008/0193860 describes a similar technique. U.S. Patent Publication No 2010/0128321 describes modulating image content for a contone image according to different polarizations (i.e., halftone screen orientations) to produce differential gloss effects. U.S. Pat. No. 7,555,139 describes adjusting line width or line spacing of a security pattern to carry data. U.S. Pat. No. 7,286,685 describes modifying a stochastic halftone pattern to incorporate a watermark.
However, these schemes require the image data to be modified using specific halftone patterns. Changing halftone patterns changes the appearance of the rendered image in more ways than simply gloss. For example, in a dot screen, the apparent densities of fine lines, as viewed by eye, vary by a certain amount depending on the angle between the line and the screen angle. In a line screen, however, the variation in apparent densities is much more significant. Fine lines substantially parallel to the line-screen angle will appear substantially solid, and fine lines substantially perpendicular to the line-screen angle will appear dotted or dashed. Using a dot screen, in contrast, fine lines either parallel or perpendicular would appear dashed.
Other schemes produce watermarks using specialized watermarking materials. Examples of such materials include colorless toners, colorless ink jet inks, and inks or toners containing specialty materials that are detectable under various special lighting conditions but that are not normally observable to the human eye. Another specialized material is an ink containing a solvent that softens fused toner. This softening changes the gloss of the softened toner. However, these schemes either require special-purpose watermarking machines or occupy space in the printer that could otherwise be used for producing visible images.
There is a continuing need, therefore, for a way of producing a gloss watermark that does not corrupt the intended appearance of the image content, and that permits producing high-quality images without specialized watermarking stations. Moreover, there is a continuing need for a way of producing such watermarks that permits the watermark to be changed from image to image.